In a radio communication system such as WCDMA (Wideband Code Division Multiple Access), LTE (Long Term Evolution), LTE-A (LTE-Advanced), and WiMAX (Worldwide Interoperability for Microwave Access) specified by the 3GPP (Third Generation Partnership Project), a base station and a mobile terminal each have a plurality of transmission/reception antennas and the MIMO (Multi Input Multi Output) technique is used to enable high-speed data transmission between the base station and the mobile terminal to be implemented.
In this radio communication system, a reference signal for measuring the channel state (hereinafter channel-state-measurement reference signal) that is configured as a signal commonly known to the base station and the mobile terminal is used to estimate the state of the channel between the base station and the mobile terminal. Based on the result of the estimation, the radio communication system adaptively controls the modulation scheme and the code rate (MCS (Modulation and Coding Scheme)), the number of spatial multiplexing (the number of layers, the number of ranks), the weight of precoding (precoding matrix, precoder), and the like, to thereby enable more efficient data transmission to be implemented. As this control, for example, the method described in NPL 1 may be used.
FIG. 11 is a block diagram showing an example where adaptive control is performed in the case of a downward channel (downlink) for transmitting data from a base station 1100 to a mobile terminal 1110.
In base station 1100, a multiplexing unit 1102 first multiplexes a channel-state-measurement reference signal (CSI-RS (Channel State Information-Reference Signal), pilot signal, known signal), which is specific to the base station, on a data signal for mobile terminal 1110 or a data signal addressed to another mobile terminal. Multiplexing unit 1102 transmits, from a transmission antenna 1103, the signal generated by multiplexing. In mobile terminal 1100, a separation unit 1112 separates the channel-state-measurement reference signal from the signal received by a reception antenna 1111. A feedback information generation unit 1113 generates feedback information based on the channel-state-measurement reference signal. Feedback information generation unit 1113 transmits the generated feedback information from a transmission antenna 1114 to base station 1100 through an upward channel (uplink, upward link). In base station 1100, a feedback information processing unit 1105 identifies the feedback information transmitted by mobile terminal 1110, from the signal received by a reception antenna 1104, and processes the identified feedback information. An adaptive control unit 1101 performs adaptive control on the data signal for mobile terminal 1110, based on the received feedback information.
Regarding the radio communication system, in an area covered by the base station, there are mobile terminals having a variety of capabilities (performance, ability). Therefore, the base station can know the capability of the mobile terminal (mobile terminal capability) to thereby enable this mobile terminal to be accommodated. As such a control method, the method described in NPL 2 may be used.
FIG. 12 is a diagram showing an example of control under which a mobile terminal reports a mobile terminal capability to a base station. A base station 1201 conveys a mobile terminal capability enquiry (UECapabilityEnquiry) 1203 to a mobile terminal 1202 through RRC (Radio Resource Control) signaling or the like. Mobile terminal 1202 conveys mobile terminal capability information (UECapabilityInformation) 1204 to base station 1201 through RRC signaling. As the mobile terminal capability information, the information described in NPL 3 for example may be used.